The risk of a blood clot in the lung, which can be fatal was almost seven times higher in IVF pregnancies, it was found, although this was still a rare event, the authors said.

Following IVF there were 3 pulmonary embolisms per 10,000 pregnancies compared with 0.4 per 10,000 natural ones.

Lead author Prof Peter Henriksson, for/of the division of cardiovascular medicine at the Karolinska Institute in Stockholm, wrote in the British Medical Journal online that although the absolute risk was low, doctors needed to be alert to the risks.

In the UK, 45,264 women had IVF treatment in 2010 and 12,714 babies were born in 2009 as a result of IVF treatment.

Prof Henriksson wrote: "Pregnant women are at higher risk of venous thromboembolism after in vitro fertilisation, particularly during the first trimester.

"The risk of pulmonary embolism in women after in vitro fertilisation was increased almost sevenfold during the first trimester, although the absolute risk was low (2-3 additional cases of pulmonary embolism per 10 000 pregnancies).

"Pulmonary embolism is, however, an elusive condition that is difficult to diagnose and is a leading cause of maternal death.

"Our finding is therefore important to health professionals dealing with women who are recently pregnant after in vitro fertilisation."

He added that women at high risk of clots should be considered for preventive clot busting drugs that can be given during pregnancy.